Traffic signals are conventionally designed to operate with incandescent filament lamps. As compared with light emitting diodes cold cathode fluorescent lamps and RF energised fluorescent lamps, i.e. low energy lamps, incandescent lamps draw more current for the same light output and for reasons of energy saving there is considerable interest in replacing existing lamps with such low energy lamps. Another reason for converting to such lamps is that incandescent lamps have a shorter life.
Because filament lamps are not durable, and because failure of a red light at traffic signals can have very serious consequences, the control circuitry used to operate traffic signals conventionally monitors the current drawn by the filament lamps to ensure that the lamps are functioning correctly.
On account of this monitoring of the current drawn by the filament lamps, their replacement with low energy lamps presents a problem because the current drawn by the low energy lamps is not sufficient to indicate to the control circuitry that the lamps are functioning correctly.
A possible solution to this problem would be to modify the control circuitry. While it would be possible to redesign the circuitry of new traffic signals to allow for the fact that low energy lamps are being used in place of filament lamps, it is not economically and commercially viable to modify existing traffic light installations in this manner.
It has previously been proposed to connect a resistance in parallel with the low energy lamps to increase the total current consumption to a level comparable with that of a filament lamp but this clearly is not a sensible solution as it defeats one of the main objects of changing to low energy lamps, namely to improve energy efficiency.
It has further previously been proposed to provide an emulation circuit for connection to a low energy lamp connected in place of a filament lamp in traffic signals, which comprises means for sensing correct operation of the lamp, and means operative when the lamp is sensed to be operating correctly to activate an auxiliary load intermittently to draw an additional current through the auxiliary load from the AC supply only between preset phase angles of the AC supply.
The above known emulation circuit suffers from the disadvantage that it does not allow for the fact that traffic signals are operated with a lower intensity by night than they are by day. This is done because a light sufficiently bright to be seen clearly by day would dazzle road users and create a nuisance by night. The reduction in light intensity is brought about by reducing the supply voltage to the lamps. The reduced voltage results in the filament lamp operating at a lower temperature which will in turn reduce its resistance and affect the current drawn from the current supply.
It is important to match the current drawn by an LED from the current supply to the current of an incandescent lamp accurately because for each aspect (red, amber, green), several lights will be energised at the same time. The controller monitors the total current drawn by all the lamps and from that determines if any of the lamps is not functioning correctly. The effect of summing the currents drawn by several lamps that are energised at the same time is to multiply by the number of lights any error in the drawn current by each lamp. If less current is drawn by each lamp, the controller will respond as if some lamps are inoperative when they are all still working correctly and, conversely, if too much current is drawn then the controller may not detect when a lamp has stopped working.